The invention resides in a method and apparatus for the arc-like shaping of profiled strands or strand sections consisting of a thermoplastic material particularly hollow window frame profile strands wherein at least the strand section to be bent is heated to a temperature permitting the shaping of the strand utilizing a heated temperature resistant support structure around which the strand section is bent and from which it is separated after being cooled and to an apparatus for performing the method.
Such methods and apparatus are known in principle for example from DE 33 22 572 C2. Here, a profile strand is softened in a heat bath, pre-shaped and then bent to its final shape using shaping templates. As heating liquid in the bath, normally oil is used whose boiling temperature is higher than the softening temperature of the thermoplastic material such as PVC. To prevent the formation of waves or the buckling at the compression side, that is the radially inner side, of the profile strand to be bent heated oil is conducted onto the strand on the bending apparatus during bending. However, the use of oil is not without problems. On one hand, vapors and deposits which may adversely affect the health of the workers are generated and, on the other hand, the shaped profile strands have to be cleaned after they cool down, which also results in problems with the disposal of the residues. A dry heating of the profile strands for example in hot air chambers as it is mentioned in DE 20 53 318, in which various heating possibilities are listed (see column 2, lines 49) would therefore be desirable. However, in practice, problems occur with heated oil operated heating baths as well as with hot air chambers since the formation of waves along the compression side of the profile strand cannot be prevented with either method, particularly with relatively tight arcs which require relatively large shape changes. Undesirable shape changes occur also at the front sides (that is, on the side disposed on the bending table and on the opposite parallel side) of the profile strands adjacent the inner profile web.
DE 27 29 548 A1 proposes to use support ledges on which the profile strands are slidably disposed during their bending while being heated. The ledges abut a profile strand essentially from all sides and consist of heat resistant profiled rubber or rubber-like materials having a relatively low bending resistance. If a PVC profile strand is supported by such a rubber profile ledge in a hot air chamber at a temperature of 110.degree. C. the PVC profile strand can be bent as desired utilizing also a template. The rubber profile can be removed after the PVC profile strand has cooled down. However, with this method, the heat-up period is relatively long, since first the rubber profile has to be thoroughly heated so that it can transmit the heat to the profile strand. It is further disadvantageous that also the side portions of the PVC profile strand are surrounded by the rubber profile. This can lead during the bending process to undesirable small pleats or waves in the compression area which are not tolerated by the consumer.
It is the object of the present invention to provide a method for heating profiled strands of thermoplastic material in a hot air chamber rapidly and bending them without undesirable changes that is without the formation of waves or buckles in the compression areas of the profile strand.